The invention relates to a predistorsion linearizer for an amplifier having a broad frequency band, in particular in the microwave range.
When the power requirements of electronic signals are large, use is made of an amplifier which is caused to operate close to its saturation power. By way of example, mention can be made of the transmission requirements of telecommunications satellites for which traveling wave tube amplifiers are used.
Operating the amplifier close to saturation gives it a response that is not linear. More precisely, when the input power is well below saturation power, then output power is substantially proportional to input power. However, as input power comes closer to saturation power, gain decreases, and takes on the value 1 at saturation. This non-linearity also affects the phase of the output signal: phase remains constant when the amplifier is operating away from its saturation zone, but for certain amplifiers (and in particular traveling wave tube amplifiers) phase decreases as power approaches the saturation value.
To remedy that drawback, it is common practice to use predistorsion linearizer devices, particularly with tube amplifiers for which both gain and phase decrease as input power approaches the saturation value. Such a device is placed upstream from the amplifier or tube to be linearized. It provides an output signal whose amplitude and phase vary as a function of input power in non-linear manner so that both the amplitude and the phase of the signal output by the amplifier vary in linear manner as a function of the signal input to the predistorsion device.
More precisely, in the linearizer device, predistorsion is applied both to gain and to phase so as to keep gain and phase constant for input power values well below saturation power and also provide gain and phase which increase with increasing input power approaching saturation power, thus compensating the decrease in gain and phase due to the amplifier.
This principle is shown in FIG. 1 where there can be seen, for example, a tube power amplifier 10 for use on board a spacecraft, such as a satellite, in order to power one or two transmit antennas (not shown) in a telecommunications system. The signal to be amplified is applied to the input 12 of the amplifier 10 via a linearizer device 14 of the predistorsion type.
The graph of FIG. 1a plots output power Ps up the ordinate axis as a function of input power Pe along the abscissa axis. In this graph, curve 16 corresponds to the linearizer device 14. It can be seen that at low input powers the curve 16 presents a linear portion 161 and that for higher levels of input power it presents a non-linear portion 162 of slope that is steeper than that of the linear portion 161.
The graph of FIG. 1b shows the way in which the amplifier 10 operates, in this case a traveling wave tube. This graph is analogous to that of FIG. 1a. Curve 18 has a first portion 181 that is linear and a second portion 182 that is non-linear for higher values of output power PS when this power comes close to the saturation power PM. In its portion 182, the slope is smaller than the slope of the linear portion 181 and it comes close to zero.
In the graph of FIG. 1c, there can be seen the relationship between the signal Pe at the input 20 of the device 14 and the signal Pxe2x80x2s at the output 22 of the amplifier 10. It can be seen that the curve showing variation in Pxe2x80x2s as a function of Pe presents variation 24 that is linear all the way to saturation PM.
The graphs of FIGS. 1a to 1c also show, in dashed lines, the variations in the phase xcfx86 of the output signals as a function of the powers Pe or Pxe2x80x2e of the input signals. In FIG. 1a it can be seen that the phase xcfx861 of the signal on the output of the device 14 remains constant (flat portion 28) for lower values of input power Pe and increases in non-linear manner (portion 30) for input powers that approach saturation power.
In FIG. 1b, it can be seen that the phase xcfx862 of the signal on the output 22 of the amplifier 10 is constant for smaller values of the power Pxe2x80x2e on the input 12 and decreases in non-linear manner (portion 34) when this input power comes close to saturation power.
In FIG. 1c, it can be seen that the phase of the signal on the output 22 remains constant (straight line 36).
Various types of predistorsion linearizers have already been studied. A first family of predistorsion linearizers is described, for example, in document U.S. Pat. No. 4,992,754 in the name of Blauvelt et al., assigned to Ortel Corp. (USA). According to the teaching of that document, the payload signal is applied to a delay line while the correction signal is generated in a parallel branch, with amplitude adjustment of the signal so as to make it equal to the non-linearity of the amplified signal, and phase adjustment of the signal so as to obtain phase opposition relative to the payload signal propagating along the delay line. The payload signal and the correction signal are added by means of a microwave combiner or coupler, and subsequently applied to the inlet of the power microwave amplifier. The non-linearities observed at the outlet from the power amplifier stage are thus substantially diminished if not eliminated.
Mention can also be made of another known type of predistorsion linearizer, e.g. as disclosed in document U.S. Pat. No. 4,068,186 to Sato et al., in the name of KDD (Japan). That linearizer is designed to work at very high frequencies to mitigate the non-linearities of a traveling wave tube (TWT) type amplifier or of a klystron. As the non-linearity generator, Sato et al. teach the use of a low power TWT. That type of amplifier introduces a small amount of delay (due to the finite propagation speed of electrons in a vacuum) in the amplified signal relative to the non-amplified signal. In order to synchronize the signals from the non-linear path and the linear path, it is therefore necessary to introduce a delay line in the correcting branch. The device of Sato et al. is relatively complicated because there are two corrector members upstream and downstream from a power splitter and a power combiner that are connected together by the two paths referred to as the linear path and the non-linear path. The linear path also comprises a microwave amplifier whose operating point is selected to be close to saturation in order to generate non-linearities which depend on the power of the input signal. In order to be able to adjust the output level from the predistorsion linearizer device without acting on the gain of the amplifier, a variable attenuator is situated at the outlet of the amplifier. The non-linear path has another member for correcting the phase/frequency characteristics. The signals from the two paths are applied to the two inputs of a second combiner coupler where they are added together (also with a phase shift xcex8 between the two signals). This combiner coupler can be a 3 dB hybrid coupler, for example, thus giving rise to a further phase shift of xcex8=xcfx80/2.
That prior art device therefore comprises two members for correcting the non-linear amplitude/frequency characteristic together with a member for correcting the phase/frequency characteristic. Embodiments of such corrector members are described in the Sato et al. document, and the text of that description is expressly incorporated by reference in the present application for its description of prior art embodiments.
The non-linear characteristics of those members add to the non-linear characteristics of the microwave amplifier. The transfer functions of all those members vary in disparate manner as a function of the frequency and the amplitude of the signals, or indeed as a function of the temperature of the components. Precautions are taken so that the non-linear signal as generated in that way is added to the payload signal in phase opposition so as to cancel the non-linearities of the power microwave amplifier (not shown), but only in a relatively narrow frequency band and in a relatively small range of inlet powers, and at a given temperature.
By acting on the characteristics of the corrector members, it is possible to obtain a transfer function having gain and phase that increase with input signal level. This is generally the desired response when it is desired to linearize a power TWT.
Another adjustment of the corrector members makes it possible to obtain a transfer function whose gain increases with the level of the input signal but whose phase decreases therewith. This is the looked-for response when it is desired to linearize a solid state power amplifier.
Another broadband microwave linearizer is described in the article by A. M. Khilla, entitled xe2x80x9cNovel broadband linearizers and their application in power amplifiers for satellite transponders and ground stationsxe2x80x9d, appearing in Proceedings Second European Conference on Satellite Communicationsxe2x80x9d, Liege, Belgium, Oct. 22-24, 1991, pp. 229-234, published by the European Space Agency (ESA), publication No. SP-332, which article constitutes an integral portion of the present application for its description of the prior art.
That document teaches the use of a predistorsion circuit in order to linearize a broadband amplifier of a space transponder in Ku band. As in the Sato et al. document, that circuit has two branches connected together at their ends by two 3 dB hybrid couplers, each introducing a phase shift of 90xc2x0. It also teaches the use of equivalent electrical lengths in the two branches, a configuration which is rather rare in the other predistorsion circuits described elsewhere in the literature.
The principle of predistorsion linearizers illustrated by the above-mentioned prior art documents is suitable for microwave applications. Nevertheless, it suffers from various major drawbacks which make implementation difficult. Amongst others, the following may be mentioned:
a first drawback lies in the fact that the operations of initially adjusting the circuits for correcting the amplitude/frequency characteristic and the corrector of the phase/frequency characteristic are often lengthy and difficult to perform, with the task being made more complicated in the usual case where the electrical lengths of the two paths are generally very different; and
a second drawback of prior art devices is that the width of the optimum operating band of the device for correcting non-linearities is limited by differences or differential variations in the electrical lengths of the two paths, either as a function of the frequency or as a function of the amplitude of the signals to be amplified, or else as a function of operating conditions, and in particular temperature.
In French patent FR 2 719 954 and its equivalent U.S. Pat. No. 5,576,660 in the name of Pouysegur, those problems are addressed with a linearizer of the kind shown in FIG. 2. That linearizer has two paths with electrical lengths that are substantially identical as in the above-mentioned Khilla linearizer. Another feature of that linearizer is that both paths are made up of identical components so as to avoid one path drifting relative to the other in the event of a change in temperature. In contrast, that linearizer is effective only over a relatively narrow frequency band.
In another patent having the same inventors as the present application: FR 2 791 197 and its equivalent EP 1 039 630 A1, a broadband linearizer is taught which operates over a bandwidth of 1.5 GHz in Ku band, but unfortunately that invention appears to us to be difficult to transpose in hybrid technology to Ka band. One of the objects of the present invention is thus to propose a broadband linearizer capable of being used in all of the various frequency bands commonly used on board telecommunications satellites, namely bands: C, Ku, and soon Ka.
Apart from the last-mentioned embodiment, the previous predistorsion devices that are capable of being used in space applications operate correctly only for bandwidths that are relatively narrow, for example 250 MHz to 500 MHz in frequency ranges lying from 10.7 GHz to 12.75 GHz.
As mentioned above, various linearizer devices are also known for amplifier tubes that are capable of operating over a broad band of frequencies, in particular over a bandwidth of 1.5 GHz to 2 GHz in Ku band. Nevertheless, those known devices present complex structures relying on a plurality of microwave integrated circuits. They are difficult to adjust and they are sensitive to temperature. In addition, they are generally expensive and bulky, which is a severe drawback for numerous applications, and in particular applications in space. Their use at high frequencies such as the Ka band around 20 GHz would appear to be difficult.
An object of the invention is to mitigate the drawbacks of the prior art.
The device of the invention makes it possible to perform linearization over a broad band of frequencies. It can be implemented at low cost and using a microwave integrated circuit technique. It can be used in all of the frequency bands presently in use for space telecommunications. Circuit adjustment is easy and reproducible, which gives rise to savings in implementation.
The linearizer device of the invention which operates in the microwave range is a broadband predistorsion linearizer for the gain and the phase of a power amplifier operating in the microwave range and comprising means for conferring gain and phase predistorsion to compensate the gain and phase non-linearities of the corresponding amplifier, said linearizer further comprising:
an inlet splitter having one inlet and two outlets;
a xe2x80x9clinearxe2x80x9d first path and a xe2x80x9cnon-linearxe2x80x9d second path, said linear and non-linear paths presenting, by construction, the same group propagation times; and
an outlet combiner having two inlets and one outlet;
the linearizer being characterized in that:
said non-linear path having a broadband non-linear element comprises a Lange coupler, each of its two accesses being grounded via respective active elements; and
said linear path comprises a Lange coupler identical to that of the non-linear path, each of its two accesses being grounded via a respective passive element.
In a preferred embodiment, said two active elements are diodes, the feed voltages therefor, via respective serial resistors, serving to adjust the curve of non-linearity gain extension as a function of the incident power. In an embodiment, said two passive elements are open-circuit segments of microwave transmission line (xe2x80x9cstubsxe2x80x9d) of adjustable length, the length serving to adjust relative phase between the linear and non-linear paths in constant manner over the entire frequency band, thus performing the function of a phase shifter.
In a preferred embodiment, at least one of said linear path and said non-linear path includes a transmission line of adjustable length so as to make it possible starting from equal group propagation time between the paths to modify said parameter and thus adjust the recombination phase as a function of frequency, and consequently adjust the frequency response of the linearizer in this manner in order to compensate the frequency response of the amplifier to be linearized.
In another preferred embodiment, at least one of said linear path and said non-linear path (preferably the linear path) includes at least one etched resistor in series or parallel with the transmission line and acting as an attenuator of value that is adjustable between a finite value and zero by short-circuiting the resistor with a foil or at least a metal wire.
In another preferred embodiment, said splitter is a 0xc2x0 Wilkinson coupler.
In another preferred embodiment, said combiner is a 0xc2x0 Wilkinson coupler.